There exist many methods for the determination of small quantities of biological substances, such as proteins, peptides, polysaccharides, lipids, steroids, hormones, viruses, enzymes, bacteria, and the like.
One type of reaction which has gained importance during recent years is the immunoassay, which has many variations. Any substance with antigenic properties, or which can form an antigen when suitably linked to another substance can be used for such quantitative determinations.
Generally the quantitative measurements in immunoassays are either carried out directly after an immunoreaction, or by measuring a "tag" attached to one of the components of the immunocomplex, i.e. the antigen or the antibody. The tags can be colored or produce color after a reaction; they may be fluorescent or radioactive. Large particles, such as latex or red blood cells which form readily discernible aggregates, can be used as well. The latter are amongst the most sensitive methods known hitherto.
One specific type of immunoassay, which makes use of spectrophotometric measurements is the enzyme-linked immunoassay, performed with enzymes such as peroxidase, alkaline phosphatase, and .beta.-galactosidase as "tags".